Technical Field
The present invention relates to a vibration transducer, and more particularly, to a technology for preventing a malfunction due to gas permeation.
Related Art
A vibration transducer is a device configured to measure an applied physical amount by detecting a change in resonance frequency of a vibrator formed on a silicon substrate. The vibration transducer is widely used for a transmitter and the like, as a MEMS (Micro Electro Mechanical Systems) device such as a pressure sensor, an acceleration sensor, an angular velocity sensor, a resonator and the like.
In the related art, the vibration transducer is formed so that a planar surface-side of a long plate-shaped vibrator having both ends fixed is parallel with a chip-shaped silicon substrate, and is configured to vibrate in an upper and lower direction of the silicon substrate. Patent Document 1 discloses a vibration transducer formed so that a planar surface-side of a vibrator is perpendicular to a silicon substrate and configured to vibrate in a horizontal direction of the silicon substrate. Thereby, the forming process is simplified and the vibration transducer can be manufactured with high precision at low cost.
FIG. 8 depicts an assembled configuration of main parts of a vibration transducer 300 disclosed in Patent Document 1, in which FIG. 8A is a sectional view and FIG. 8B is a plan view of an activation layer part.
As shown in FIG. 8, the vibration transducer 300 has a structure where an activation layer 320 of silicon is processed to form a vibrator 330, a first electrode plate 341 and a second electrode plate 342 on an SOI substrate having a BOX layer oxide film 311 inserted between a silicon substrate 310 and a surface silicon layer (below the activation layer 320), an insulation oxide film 360 and an upper polysilicon layer 350 are additionally stacked and a shell 351 is formed by the upper polysilicon layer 350.
The first electrode plate 341 and the second electrode plate 342, which are fixed electrodes, are formed to sandwich the vibrator 330 therebetween, and a vacuum chamber 370 is formed around the vibrator 330. In addition, an electrode is formed at an end-side of the vibrator 330, too, and functions as a vibrator electrode plate 331.
Unevenness parts 380 for attachment prevention are formed on facing surfaces between the vibrator 330 and the first electrode plate 341 and between the vibrator 330 and the second electrode plate 342.
In the above configuration, when an excitation signal is applied to the first electrode plate 341 at a state where a bias voltage is applied between the vibrator electrode plate 331 and the second electrode plate 342, the vibrator 330 vibrates at a resonance frequency, and an electrostatic capacity between the vibrator 330 and the second electrode plate 342 changes, so that a current is output from the second electrode plate 342. The output current is converted into a voltage at a current-voltage conversion circuit using a calculation amplifier (not shown) and a frequency is detected by an output sensor. At this time, a combination of the voltage and signal to be applied to each electrode plate and the number of the electrode plates can be changed.
When different pressures are applied to the upper and lower parts of the vibration transducer 300, distortion is caused in the vibration transducer 300 and the resonance frequency of the vibrator 330 changes in correspondence to a magnitude of the distortion. For this reason, it is possible to obtain a difference between the applied pressures on the basis of the change in detected frequency.